The present invention relates to cardiac pacemakers, and more particularly to an implantable cardiac pacemaker having means for detecting and terminating a cardiac arrhythmia, such as a tachycardia. Even more particularly, the present invention relates to applying a scanning stimulation pulse to the heart during each cardiac cycle in accordance with a prescribed sequence, i.e., a sequence wherein the timing of the stimulation pulse relative to the cardiac cycle varies slightly from one occurrence to the next. Through this process, the time region between successive heart beats is scanned for the purpose of finding a time where the stimulation pulse terminates the cardiac arrhythmia. Once the cardiac arrhythmia is terminated, and upon the next occurrence of a cardiac arrhythmia, the scanning stimulation pulse is applied to the heart beginning at a location in its prescribed sequence that is backed up a prescribed amount from the previous location in the prescribed sequence where the stimulation pulse successfully terminated the prior cardiac arrhythmia.
Cardiac pacing, i.e., the selective application of a simulation pulse to the heart, has been utilized for many years for the purpose of terminating intrinsic atrial and/or ventricular arrhythmias. A tachycardia is an arrhythmia in which the heart beats very rapidly, e.g., above 150 beats per minute. The principle underlying cardiac pacing for the purpose of terminating a tachycardia is that if a pacemaker stimulates the heart at least once shortly after a heartbeat, before the next naturally occurring heart beat at the rapid rate, the heart may revert to sinus or natural rhythm. This is because a tachycardia is often the result of electrical or other feedback within the heart. That is, a natural heartbeat results in the feedback of an electrical or other stimulus which may prematurely trigger another beat. By injecting a stimulation pulse within the cardiac cycle, the stability of the feedback loop is disrupted, and the heart may revert to a sinus (natural) rhythm.
The difficulty in using a stimulation pulse to terminate a tachycardia lies in determining exactly when the stimulation pulse should be applied. It must be applied shortly after a heartbeat and prior to the time when the next premature beat would otherwise occur. Said another way, it must be applied at a certain time within the cardiac cycle. As the cardiac cycle is manifest by, e.g., the occurrence of R-waves (with each R-wave representing the depolarization of ventricular muscle tissue), the stimulation pulse must therefore be applied at a time between successive R-waves. However, there is usually only a short period of time, hereafter termed the "region of susceptibility", somewhere between successive beats (R-waves) during which the generation and application of a stimulation pulse has a high probability of successfully terminating a tachycardia. Unfortunately, this region of susceptibility varies not only from patient to patient, but from day to day with the same patient. Moreover, for any given patient on any given day, the "region of susceptibility" within the overall tachycardia cycle is relatively short, and may vary even during a single tachycardia episode. (Note, as used herein, the term "region of susceptibility" refers to that narrow time period within a given arrhythmic cycle during which the heart is susceptible to reverting back to a sinus rhythm through the application of a stimulation pulse. This time period may be thought of as an arrhythmia termination window. The terms "region of susceptibility" and "termination window" are used interchangeably herein to refer to this narrow time period.)
In order to increase the likelihood that a stimulation pulse will be applied during the region of susceptibility, and therefore be successful in terminating a tachycardia, or other arrhythmia, it is known in the art to use several techniques to hunt for and find the region of susceptibility. Typically, heretofore, the manner in which the "hunting" for the region of susceptibility has been accomplished follows one of two approaches: (1) "shotgunning" the region between successive R-waves with a burst of pulses; or (2) scanning successive single or multiple stimulation pulses through a scanning window.
Shotgunning is premised on the theory that by providing a burst of pulses, the likelihood that at least one of the pulses will fall within the region of susceptibility of the heart significantly increases. If not, the rate of the pulses within the burst (i.e., the time spacing of pulses within the burst), or the position of the burst relative to the cardiac cycle, is modified slightly and reapplied to the heart during a subsequent heart cycle. This process continues until the region of susceptibility is found, and the tachycardia terminates. Once found, the timing associated with the successful burst may be stored and used as the starting point for applying a new burst of simulation pulses to the heart upon the next occurrence of a tachycardia. U.S. Pat. Nos. 4,398,536 (Nappholz et al.); 4,406,287 (Nappholz et al.); 4,408,606 (Spurrell et al.); 4,541,430 (Elmqvist et al.); and 4,561,442 (Vollmann et al.) are representative of this shotgun (burst pacing) approach of terminating a tachycardia.
The other way to find the region of susceptibility is to define a scanning window, e.g., a period of time between successive heart beats, through which one, two or more successive stimulation pulses are applied in a controlled sequence. The controlled sequence may be, e.g., beginning at one end of the scanning window and scanning in a controlled manner towards the other end of the window with each successive stimulation pulse or group of pulses. Hence, the stimulation pulse scans through the scanning window looking for the region of susceptibility. For example, as taught in U.S. Pat. No. 4,312,356, issued to Sowton et al., a pacer is provided wherein the sensing of a tachycardia triggers a stimulation pulse having a known, and somewhat arbitrary timing, relative to the tachycardia cycle. This stimulation pulse is thus applied to the heart at a time within the cardiac cycle that represents a first guess of the location of the region of susceptibility. If the stimulation pulse is not successful in terminating the tachycardia, i.e., if the first guess is incorrect, then a subsequent stimulation pulse is provided so as to be issued later or earlier relative to the timing of the unsuccessful stimulation pulse. In this trial-and-error manner, the region of susceptibility is eventually located, and the tachycardia is terminated. Unfortunately, this approach may require a significant "hunting" time before the region of susceptibility is located.
In order to shorten the hunting time, it is also known in the art to store the timing associated with the last successful stimulation pulse(s). This timing is then used as a starting point when the next tachycardia occurs. In this manner, the "hunting" time is believed to be significantly reduced. U.S. Pat. Nos. 4,390,021 (Spurrell et al.) and 4,427,011 (Spurrell et al.) are representative of the prior art approaches taken using a pulse that hunts for the region of susceptibility by scanning through a scanning window. U.S. Pat. No. 4,577,633 (Berkovits et al.) accomplishes essentially the same result (of providing a single scanning stimulation pulse that hunts for the region of susceptibility) by continuously shortening the pacemaker escape interval with each subsequent beat, for a predetermined number of beats, by a small programmable decrement.
It is also known in the art to combine shotgunning (burst pacing) and single-pulse scanning as selectable options within a single pacemaker, as is taught, e.g., in U.S. Pat. No. 4,726,380 (Vollmann et al.).
Burst pacing has been very successful in terminating cardiac arrhythmias, and was initially the preferred approach. However, experience has shown that its continued use may result in a significant incidence of arrhythmia acceleration, and a corresponding worsening of the hemodynamic state of the heart. (As used here, "hemodynamic" refers to the ability of the heart to efficiently perform its function as a pump.) As a result, the stimulation pulse scanning techniques have been favored in an attempt to lesson the risk of arrhythmia acceleration while maintaining a high efficacy for arrhythmia termination. Scanning of the stimulation pulse has been necessary in order to account for the variations in the exact timing of the "region of susceptibility" (or tachycardia termination window) that regularly occurs in any given patient. For example, changing blood levels of antiarrhythmic drugs, postural changes, catecholamine levels, and numerous other factors which affect the conduction velocity and refractory characteristics of cardiac tissue, may result in a movement of the region of susceptibility for any given patient.
Scanning may involve the use of one, two, or three pulses, or a burst of pulses. Typically, after each pacing attempt which is unsuccessful in terminating the arrhythmia, one or multiple of the cycle lengths of the pulses are reduced, and the new critically timed intervals are thereafter used. If this attempt is unsuccessful, the reduction is continued. In this way, the pulses are delivered sooner and sooner within the cardiac cycle, and the time between pulses (if multiple pulses are used) becomes shorter and shorter. Depending on the total range of cycle lengths to be reduced, the amount of reduction within each attempt, and the number of pulses and sequence to be reduced, an entire cycle of scanning may be a long process. Hence, it is known in the art, as mentioned, to use the memory of the device to automatically restart any new scanning sequence at the intervals which were last successful for termination. Unfortunately, however, if the region of susceptibility (termination window) has shifted in a direction that causes longer intervals for termination, this starting point of the scanning sequence may require an entire scan to be completed before finding the termination window. What is needed, therefore, is an arrhythmia termination system that prevents long periods of time in hunting for the region of susceptibility.